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Rational Descriptive Classification of Duricrusts EARTH SCIENCE JOURNAL Vol. 3, No. 2, 1969 RATIONAL DESCRIPTIVE CLASSIFICATION OF OURICRUSTS G.H.DURY Department of Geography and Geology, The University of Wisconsin Abstract The term duricrust appears to be extending itself to include calcareous, gypseous, and saline crusts, in addition to crusts composed dominantly of silica and/or of sesquioxides of iron and aluminium, with. -or without significant contents of dioxides of manganese or titanium. This latter group can be distinguished as duricrusts proper. Its nomenclature is highly confused, and its classification, in writings in the English language, defective. The relevant problems can be resolved, at least in considerable part, by the introduction, adaptation, and extension of modern terms current in tropical pedology, to give a descrip- tive classification free of genetic implications. When content of Si02, AbOg, and Fe20g is used as a primary basis for the classification of duricrusts proper, plots on a ternary diagram justify the recognition of seven named types in the fersiallitic range. The nomenclature and c1assHicati.on .of duricrusts, as they have developed in English. are undeniably cDnfused: On a far smaller and much less complex scale, the situation resembles that faced by the authors of the 7th Approximation to the comprehensive classification .of soils (V.S. Department Df Agriculture, 1960). Perhaps a more apt comparison would be with the difficulties in the classification .of climate which Thomthwaite (1931. 1948) set out to resolve. Both in pedology and in climatology. the authors of the new classifications aimed at generally- applicable systems, relying on consistent criteria thrDughDut, and intrDducing new terminologies which relate to precise definitiDns and which are free from the unwanted imprecision and associations of previDus terminologies. Although it does not yet seem pOlssible to dO' as much for the naming and classification of duricrusts, the dubious and at times misleading connotations Df some of the terms in current use, plus seriDus deficiencies in what may be called the first apprDximative classificatiDn in English, point the need fDr an attempt to produce some kind .of taxonomic order. As is well known, the term duricrust was .originated by WODlnough ( 1927) for application tD surficial crusts in Australia. These had previously been variDusly described as . laterite, lateritic bauxite., bauxitic laterite, bauxite, concretionary ironstone, surface quartzite, and Desert Sandstone, according to assumed content .or origin and/or observed litholDgy and mineralDgy. The names listed cannot however be regarded as even a first appro~imation to a ratiDnal classification of duricrusts. Those in the laterite-bauxite range, quite apart frDm the uncertainties about chemical and mineralogic constitution which they entail, fail to make the necessary distinction between chemistry and mineralogy on the one hand and physical characteristics .on the other. The terms lateritic, bauxitic, &c., crusts would seem to be called fDr, even thDugh thick and well-cemented crusts can grade laterally into thin surface litters .or SUbSDil layers .of fragments, pisoliths, and nodules, and may wedge 0'ut altDgether. By nO' means all bauxites are sufficiently well cemented to' form crusts, and many appear to consist of transported material as opposed to residues in situ. Laterite, on anything like a strict definitiDn, need not be and usually is not indurated at all. The prevalent .obstacles to arriving at a strict definition of laterite need not concern us here (for discussion, see for 77 example Prescott and Pendleton, 1952; Maignien, 1966), since the term does not enter into the proposed classification. It is however worth observing that Buchanan's original laterite of 1807 came from the mottled, not the uppennost, part of a deep-weathering profile; that the confused associations of the word laterite are such as to cause some workers to recommend its complete abandonment; and that the description of highly siliceous deep-weathering profiles in Australia as lateritic has led to highly unlikely hypotheses of their origin (for discussion of this last topic, see Langford-Smith and Dury, 1965). What may be regarded as a first approximation to the classification of duri- crusts originated with Lamplugh (1902, 1907) who introduced the terms calcrete, silcrete, and ferricrete. To these have subsequently been added gypcrete and salcrete. A parallel list replaces -crete by -crust (Table I, cols. i. ii). It seems unnecessary here to identify the points of introduction or the original authorship of the additional terms: for present purposes, they may be accepted as in widespread use, their currency being taken as a guarantee that they are thought useful. Terms in -crete and terms in -crust are not wholly interchangeable. As Wopfner and Twidale (1967) point out, it is appropriate to distinguish between well-cemented and massive gypcrete and loosely-cemented to powdery and impure gypseous crusts. Furthermore, the various -cretes can occur outside the range of -crusts: ortstein for instance can be regarded as a form of ferricrete. A weakness of applying -crete to the material of duricrusts is that the element refers merely to agent of cementation and not necessarily to major bulk content. Thus, while calccrusts are assumed to be composed largely of calcium carbonate (under analysis, 90% or more is common), calcretes need not be. A ready example is provided by the calcite-cemented outwash gravels in the north of the London Basin, where most of the material involved consists of sHica in the form of flint. A general defect in both lists is that they make no provision for highly aluminous duricrusts, presumably because a quite small iron content in the ferricrust range can result in a strong hematitic coloration (cf. however Maignien, 1958, who distinguishes between aluminous and ferruginous latedtes). At the same time, the separation of silcrusts from ferricrusts is useful, as recognising the occurrence of highly siliceous duricrusts, and as superseding the block term lateritic. It is probably too late to debate whether or not calccrusts, gypcrusts, and salcrusts should be classed as duricrusts at all, since the term duricrust appears to be extending itself to embrace them: there is no purpose in trying to turn back a linguistic tide. Equally, the force of usage is probably strong enough to' prevent the substitution of calccrete, the logical accompaniment of calccrust, fDr the original calcr.ete. There can however" be- no" objection to styling crusts in the si1crust- ferricrust range of the first approximation duricrusts proper (cf. Table 1). The difference of origin between duricrusts proper and other types of crust needs constantly to be borne in mind. The latter are associated withsubhumid to arid climates, whether those of the present or those of the past. Many of them have strong affinities with evaporites, even when they do not result directly from the drying of pluvial lakes or from the evaporation of playas or of emergent saline groundwater. However, the name evaporation crusts is not entirely suitable: fOif example, Coque (1962) has identified crusts formed of wind,.laid gypsum. Again, calccrusts can alternatively originate as calcareous soil horizons, including calcareous duripans (7th Approximation, 55-59). The fact that crusts of this subtype form mainly below the level of the ground surface is no necessary obstacle to their classification as crusts, since duricrusts proper originate, at least in very· large part, in precisely the same manner. Gypsic. and for that matter natric and salic soil horizons also (7th Approximation, 45-46. 59-60) seem at most to rise to the status of fragipans (ibid., 56-57) not resisting 78 stripping sufficiently to constitute crusts: in the present context, therefore, they may be disregarded. In respect of calccrusts, it is not prDposed here to review the contrOoversial aspects of the origin of soil carbonate, Oor to discuss the various names which it has been given (e.g., caliche. kunkar. travertine). The tenn calcsilitic crust, mentioned here in anticipation of an outline of the second approximation. is prDvisionally inserted in the classification as connoting calccrust or other surficial limestone such as calcarenite, which has undergone silicification. Investigation is required both into the status of this type and intO' its place in the classification of silicified limestone in general. Duricrusts proper are called, by a number of authors, weathering crusts, but this usage is not wholly defensible. The major extent of duricrusts prDper admittedly seems to' be associated with deep-weathering profiles, of which the duricrusts fOorm part; but scarpfoDt, detrital, and valley-bottom crusts are well documented from numerDUS areas (cf. de Swardt, 1964; Maignien. ap. cit. 1966). where they can either be assDciated with deep weathering or merely result from the cementation of colluvium, terrace deposits, and valley fills. cDnstituting the forms widely called detrital laterites (or lateritites) and ground water laterites. Much obviously depends on the highly variable interplay Df weathering. vegetation. climate, erosion. and time. In this connection. attention may be drawn to a pDtential source of specific confusion. With some writers. primary laterites are those of a residual erosion-platform, secondary laterites those formed below the level of a wasting duricrust cap, and fDrmed from material fallen or leached from it; with others (e.g., Mohr and
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